By their constant contact at the cylinder barrel, piston rings are subject to constant sliding wear. This is expressed both in the abrasive degradation of the piston ring surface or its coating and in the partial transfer of material from the cylinder surface to the piston ring surface and vice versa. It is possible by using adapted coatings to minimize these negative affects. Consequently, particle-reinforced hard chromium layers exhibit better resistance to abrasion than uncoated or nitrated rings (see EP 217126 B1), but also better than conventional hard chromium layers or plasma sprayed layers on a molybdenum base. Nevertheless these coatings, too, lapse into the borderline region of their performances, because of the increasing pressure and temperature parameters in modern internal combustion engines. Therefore, new coatings are required that provide even less abrasion and higher resistance to abrasion versus the currently available ones. Ceramics are suitable in principle as materials that can fulfill these requirements. They have excellent resistance to abrasion and, because of their non-metallic bonding properties, have very low tendency to adhere in comparison to metal alloys.
Ceramics can also be applied directly to piston rings using various coating methods. So, for example, they can be directly deposited using vaporization methods (PVD or CVD). The drawback here is that the amount of material deposition per unit of time for this application are much too low and are therefore uneconomical.
Plasma spraying, on the other hand, provides relatively high deposition rates but the coatings are generally subjected to tensile stresses, whereby they run the risk of cracking and breaking out. This is aggravated generally by the very brittle character of the ceramics.
Thermal spraying techniques using nano-crystalline hard metals (nano-crystalline=1 to 100 nm) are exhibiting increasingly positive results. As early as the late 1980s nano-carbide reinforced materials were being processed in layers using vacuum plasma spraying techniques. Higher hardnesses in the layers produced with comparatively lower hard material content can be obtained using this method. The coatings exhibit clearly higher ductility and resulting higher impact resistance than conventional reinforced materials. However, high-velocity flame spraying technology first made it possible to create powder morphologies also in the layer. Nano-oxide reinforced metals are primarily sprayed using high-velocity flame (HVOF) spraying. The spray powders are manufactured using high-energy milling. This process is particularly interesting for thermal spray powders, because it results in a number of special powder properties. The density of stacking errors, defects and deviations are increased on the surface of the powder by the crushing and milling process, whilst particle sizes can be reduced to nano-crystalline dimensions. These permanently freshly generated surfaces are characterized by high activity, so that even high-strength oxide-metal and carbide-metal combinations can be produced.
It is therefore desirable to combine the good tribological properties of the ceramics with the good mechanical properties of metals. It is therefore conceivable, for instance, to introduce ceramic particles into a metallic matrix, whereby ductile and viscous compounding of the hard and in part brittle ceramic particles is possible. The ceramic particles can then, with appropriate exposure at the surface, assume the tribological roles, while the metal matrix can take on the mechanical loads and breakdown stresses by deformation, if necessary.
Such a combination concept is already being realized today. So, for example, powdered hard metals (WC—Co) or cermets (NiCr—CrC) are being processed to layers by means of thermal coating processes. The basis for this is either a powder mixture or a compound powder. As a rule, however, mechanical mixtures provide the lowest coating quality, since in this case compound formation occurs only in the coating process and the hard materials must be relatively large due to their required fluidity. Compound powders are generally manufactured by agglomeration to so-called micro-pellets. In this process micro-fine starting powder is processed in a spray-drying process to powders that can be processed; in other words, primarily to fluid powders. In order to increase the strength of the agglomerate or to obtain certain agglomerate densities, they are in most cases subsequently sintered. Another possibility for manufacturing compound powders is mixing the components with subsequent sintering to the block. In this case, the powder is obtained by crushing and milling the block. Furthermore, compound powders are manufactured by enveloping, wherein, for example, a hard material powder is chemically or physically coated with a metallic element—so-called cladding—wherein fine metallic powders are adhered to the hard material core by a spray-drying process.
Characteristic of the manufacturing of common compound powders is that the formation of the compound in the powder generally requires a sintering process, because the powder can otherwise degrade into its starting components in course of the coating processes and lose the advantageous compound effects in the coating. This is all the more important the greater the processing forces during coating. These are especially high in the high-velocity spray methods, wherein the powder is processed in a supersonic gas current. Moreover, optimal binding between the ceramic and metallic binding phase is required for fulfilling the tribological tasks and can be obtained particularly by chemical-metallic binding.
The drawback in the required sintering is that on the one hand the economy of the powder is reduced and on the other hand the starting ingredients must be capable of being subjected to sintering. This is evident especially in the case of WC—Co combinations but is absent, however, in the case of those combinations that are of interest for economical and tribological reasons comprised, for example, of metallic binders and oxide-ceramic hard materials. Therefore, until now, such powders could not be used successfully for thermally coating piston ring surfaces.
A support for thermally coating metal parts such as, for instance, piston rings and cylinder barrels, is disclosed in DE 19700835 A1. The composite powder used in said document is a mixture of carbides, metal powder and solid lubricants which is processed using a high-velocity oxy fuel spraying method to a self-lubricating composite layer. The composite particles comprised of CrC and NiCr are mixed with the solid lubricants for creating the composite powder.
The drawback in this type of production of the composite powder according to DE 19700835 is the fact that in order to obtain the necessary fluidity, as a condition of processing using the high-velocity flame spray method, relatively coarse granular particles must be formed. In the case of these mixed, non-spherical composite powders the granule size of the solid lubricant particles must be >20 μm so that the composite powder has the necessary fluidity for spraying in the high-velocity flame method. These coarse particles require a concentrated accumulation of solid lubricant phases in the coating and this, in turn, has negative ramifications on wear, since the coarse and thus relatively large solid lubricant zones can break out and due to their size are available only punctally as a lubricant.
It is, therefore, the object of the present invention to expand the coating materials in terms of powder technology so that tribologically optimized surfaces are created for the piston ring.
Therefore, a thermally applicable coating composition for the bearing surfaces of piston rings etc. is provided, wherein said composition can be manufactured using mechanically alloyed powders.
According to the invention, said object is achieved by the coating and by the piston ring as described and claimed herein.